In German Patent No. 2,627,880, there is described a nozzle design for forming atomized sprays in which a gas medium and a liquid medium are combined in a mixing chamber and then expelled from the nozzle as atomized liquid or as tiny gas bubbles, depending on the relative proportions of the liquid and gas and whether sprayed into a gaseous or liquid medium. The atomization results from a considerable drop in pressure as the two-phase mixture leaves the nozzle. The nozzle is based on the principle that a properly-formed two-phase mixture has an effective sonic velocity that is only a fraction of the sonic velocity of the two pure phases. For example, the speed of sound for clean water under normal conditions is 1500 m/s and for clean air approximately 330 m/s. The speed of sound of a defined two-phase mixture is approximately 20 to 30 m/s. This nozzle design has many attributes, including lower operating pressures, lower pressure drop, reduced velocities, reduced air consumption and reduced orifice abrasion.
However, the nozzle consists of a single orifice which has many shortcomings. For example, if a large duct is to be completely filled with fine liquid spray, the 12.degree. to 15.degree. spray angle generated by the single orifice may require placement of the nozzle many meters back in the duct or the use of a multiple number of individual nozzles to achieve the objective.
In the nozzle design described in the above-noted German Patent, the liquid feed is effected through the same pipe as the spray is ejected from, while the gas is fed from the side to a chamber which surrounds and communicates with the liquid feed through a plurality of openings in the liquid feed pipe just upstream of the orifice, so as to form the two-phase mixture therein. This feed arrangement often is unsuitable for the feed lines available and the intended end use.
U.S. Pat. No. 4,893,752, assigned to the assignee hereof, describes a number of novel nozzle designs intended to overcome the drawbacks of the nozzle design of German Patent No. 2,627,880 by providing a multiple number of orifices communicating with a single source of both liquid and gas and arranged to spray in different directions away from the nozzle. These nozzle designs can be termed "cluster nozzles".
The cluster nozzle designs of U.S. Pat. No. 4,893,752 were developed to serve various applications for the nozzles in terms of quantity of liquid to be sprayed from a single nozzle, angle of spray pattern required, density of spray within the spray pattern, spray droplet size distribution desired, whether a clean liquid or a slurry was to be sprayed, and where the spray was to be introduced to the system.
More demanding requirements now are being required to be met for the cluster nozzle designs. These requirements generally relate to the quantity of liquid to be sprayed from a single nozzle, the density and angle of the spray pattern to be delivered and the droplet size distribution to be generated.
An increase in the amount of liquid to be sprayed can be met generally by an increase in size of the orifices in the nozzle or by adding more orifices of the same size. While single orifice nozzles may range up to 35 mm in size (I.D. of the orifice), the standard nine orifice cluster nozzles (designed as seen in FIGS. 3 and 4 of U.S. Pat. No. 4,893,752) with orifices larger than 8 or 9 mm do not perform as well as a similar nozzle with 8 mm or smaller orifices. This observation, in effect, has placed a limitation on the quantity of liquid that can be effectively sprayed from a single nozzle. The main deficiency observed, say for a nine orifice nozzle where the orifices were 10 mm, was a very non-uniform distribution of liquid emanating from each of the orifices.
It is also observed that, as more orifices are used in the cluster nozzles of U.S. Pat. No. 4,893,752, the orifices must be smaller yet if uniform spray patterns are to be obtained. Thus, a 16.times.6 mm cluster nozzle was designed for a particular application but did not produce the degree of spray uniformity desired. A nozzle with 16.times.6 mm orifices installed in three concentric rings about the axis of the nozzle was built with each orifice preceded by a chamber where the liquid was introduced at the end opposite the orifice, via a separate liquid chamber, into a mixing section where the atomizing gas was introduced radially into the liquid flow through a plurality of orifices which were fed gas from the chamber communicating with a source of gas. The gas and liquid form a two-phase mixture within the mixing chambers which is subsequently ejected through the orifice whereupon a spray is produced. In this case, we were able to produce a wider spray angle while significantly increasing the density of spray droplets within the spray pattern. However, we were still able to detect some degree of variability emanating from the orifices.
In U.S. Pat. No. 4,893,752, there is also disclosed a two-phase nozzle wherein a single mixing chamber is employed wherein the liquid and gas streams are joined to form a two-phase mixture, the mixture then being directed to an array of orifices located as desired at the delivery end of the nozzle (see FIGS. 5 and 6 of the U.S. Pat. This structure is claimed in U.S. Pat. No. 5,025,989 divided out of U.S. Pat. No. 4,893,752). Within certain constraints, this embodiment of the cluster nozzle has been found to produce excellent sprays which are comparable to the sprays delivered from nozzles where a separate mixing chamber preceded each individual orifice (as in FIGS. 3 and 4 of U.S. Pat. No. 4,893,752). However, the constraints experienced were similar to those found for the standard cluster nozzle, i.e. one with individual gas-liquid mixing chambers for each orifice, namely, less than perfect sprays emanating from each orifice as their size and the amount of liquor being sprayed increased.